Molecular and Cell Biological studies on Intracellular Symbiosis

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1. Symbionin, an aphid endosymbiont-specific protein, is an energy-coupling protein which undergoes autophosphorylation in the presence of high-energy phosphate compounds and transfers its phosphate group to other acceptor compounds. While this property of symbionin is reminiscent of that of histidine protein kinases in the tow-component system that transduces environmental signals in E.coli, no considerable homology between the two proteins is apparent.2. It has been suggested that symbionin is phophorylated at His-133, where GroEL has Ala instead of His. Since between the two proteins the overall amino acid sequence is highly conserved, it is all the more conspicuous that symbionin has created His at this position as the result of three consecutive base substitutions of a codon for Ala in GroEL.It is possible that symbionin has acquired a new function through positive selection in favor of His at this position, suggesting that adaptive evolution is in action even at molecular level.3. While in the groE operon the expression of GroES and GroEL are comparable with each other, in the aphid endosymbiont the expression of GroES homolog is disproportionately less than that of symbionin.4. In the synthesis of essential amino acids by the endosymbiont using glutamine, glutamine is first converted into glutamate and ammonia in the bacteriocyte and then absorbed by the symbiont.5. One of gut microbes of aphid resembles E.coli, but prefers lower temperatures than does the latter. Comparison of structure of several genes suggested that the gut microbe is phylogenetically closer to the endosymbiont than is E.coli.